1. Field of the Invention
This invention relates to a method for acquiring azimuth information using signals transmitted from GPS (Global Positioning System) satellites.
2. Description of the Prior Art
Previously, positioning information such as latitude, longitude, altitude and GPS (Global Positioning System) time were readily obtainable from signals transmitted by GPS satellites but azimuth information could not be acquired.
The inventor therefore developed a method for acquiring azimuth information by use of a pair of planar patch antennas (Japanese Patent Application Nos. 2000-91362 and 2001-93964).
This method for acquiring azimuth information includes a step of disposing a pair of planar patch antennas back-to-back, parallel to each other and vertical, whereby each planar patch antenna forms a sky coverage area of antenna sensitivity that is a sky quarter-sphere in the direction the antenna faces; a step of causing receiver units connected to the respective antennas to extract strength values of all received GPS satellite signals; a step of discriminating based on a comparison of the extracted signal strength values the antenna in whose sky coverage area the GPS satellite that transmitted each signal is present; a step of arranging the results of the step of discriminating areas of satellite presence in a ring-like sequence; and a step of determining or limiting an azimuth of a measurement direction based on information contained in a ring-like discrimination results sequence.
In order to implement this method for acquiring azimuth information in a commercially available GPS receiver, the inventor further developed a GPS receiver including a data transmitting unit, a data receiving unit and a data processing unit (Japanese Patent Application No. 2000-364605).
This made it possible to readily acquire azimuth information by disposing a pair of planar patch antennas back-to-back, parallel to each other and vertical, disposing a pair of GPS receivers so that the data transmitting unit and data receiving unit of each GPS receiver faces the data receiving unit and data transmitting unit of the other, thereby enabling GPS satellite data received by one GPS receiver to be transmitted to the other GPS receiver, and processing the two sets of data with the data processing unit.
Azimuth information acquired from GPS satellite signals is more reliable than azimuth information acquired using a compass that is affected by magnetic fields.
However, the foregoing method for acquiring azimuth information proposed by the inventor requires two planar antennas to be disposed in parallel. Therefore, at a location where only a limited azimuth portion of the overall sky can be used because of the presence of high-rise buildings to the front or rear, it is difficult to obtain an azimuth limitation result that limits the measured direction to a relatively narrow azimuth range.
Moreover, the fact that in the earlier proposed method for acquiring azimuth information the two planar antennas have to be disposed in parallel makes them difficult to install at the nose of a land mobile unit with a streamlined nose.
The present invention was accomplished in light of the foregoing circumstances and has as an object to provide a method for acquiring azimuth information that enables acquisition of azimuth information provided that a sky coverage area enabling reception of a signal from at least one GPS satellite is available.
This invention achieves this object by providing a method for acquiring azimuth information comprising:
a step of vertically disposing a pair of planar antennas each having a hemispherical antenna pattern at an angle to each other such that their sky coverage areas partially overlap;
a step of detecting the angle formed between the antennas;
a step of causing a GPS receiver connected to each antenna to attempt reception of signals transmitted by GPS satellites in its sky coverage area;
a step of comparing the satellite signals acquired and discriminating in which of three separate sky coverage areas the satellite that transmitted each signal is present;
a step of in each of the three areas creating a clockwise series of satellite azimuths and extracting the azimuth of the first term and the azimuth of the last term; and
a step of limiting the azimuth of a direction based on the azimuth of the first term and the azimuth of the last term in at least one area and the angle formed between the two antennas.
Thus in the method for acquiring azimuth information according to the present invention, in a situation where blocking by a mountain, building or other such ground feature is pronounced, allocation of antenna sensitivity to the whole sky can be avoided and the foregoing tendency for the spread of the azimuth limitation result to be wide can be overcome to provide a more desirable azimuth limitation result.
Moreover, owing to the placement of the two planar antennas to form a certain angle in the method for acquiring azimuth information according to the present invention, the pair of antennas can be readily installed at the nose of a streamlined land mobile unit with the angle set to match that of the streamline form, so that azimuth information can be acquired during travel.
The above and other objects and features of the invention will become apparent from the following description made with reference to the drawings.